MOSCOW, Dec 31 (Reuters) -- After announcing his resignation on Friday, Russian President Boris Yeltsin handed over to his acting successor one of the most important symbols of power in Russia: the briefcase with codes to launch nuclear missiles.

Presidential spokesman Dmitry Yakushkin was quoted by Interfax news agency as saying Yeltsin handed over the "nuclear briefcase" to Acting President Vladimir Putin shortly before finally leaving his Kremlin office at 1100 GMT.Yeltsin received the briefcase from Soviet leader Mikhail Gorbachev, who resigned on Christmas Day in 1991. Yeltsin parted from it only once during his term in office--in 1996, when he underwent heart surgery and turned over his powers briefly to then Prime Minister Viktor Chernomyrdin.

"The nuclear button is an effective mechanism to control Russian nuclear forces and also a symbol of the presidency," former Yeltsin press secretary Sergei Yastrzhembsky said when asked to describe the device.

The briefcase is carried behind Yeltsin by an officer dressed in a distinctive black navy uniform which makes it easy for the president to single him out in a crowd. But all information about it has been classified until lately.

A senior parliament member, Alexei Arbatov, has described the nuclear button as the first link in a chain of commands ending in onboard cruise computers of nuclear missiles.

"The nuclear button...transmits presidential sanction for the use of nuclear weapons to command centres where general staff officers are on duty around the clock," said Arbatov, an expert on national security with close ties to the Kremlin.

"On receiving a coded signal, officers...using appropriate codes, determine that it was the president who sent it, rather than someone else."

When the authenticity of the presidential message is confirmed, duty officers open safes containing their own codes and send them to missile launch pads and nuclear submarines.

"Then the codes are installed (in onboard cruise computers), launch keys are turned and the missiles blast off," he said.

Russia, which inherited the nuclear forces of the former Soviet Union, has some 6,000 strategic nuclear warheads, enough to destroy life on earth several times over, as well as stocks of medium- and short-range weapons.

Yeltsin himself reminded the world of this just weeks ago on a visit to China, when he confronted Western criticism of Russia's military action in Chechnya by saying U.S. President Bill Clinton had forgotten Russia was a nuclear power.

According to NTV commercial television, some 30 people are involved in handling the nuclear button network, run jointly by the Defence Ministry and the secret services.

Arbatov has said the defence minister has a similar nuclear button but the president did not need to coordinate his orders with the military chief.

"The first order (from the president) does not need a confirmation by the second," Arbatov said. He did not make clear whether the defence minister would need the president's authorisation to use his nuclear button.